Which of the following causes motors to burn out and circuit breakers to trip?

Harmonics can lead to various issues in electrical systems, including overheating of motors and premature tripping of circuit breakers. When harmonic currents flow through inductive components like motors, they generate additional heat due to the increased current flowing relative to the fundamental frequency. This can cause motors to operate inefficiently and ultimately fail if the overheating is significant enough. Additionally, harmonics can create a misalignment in the voltage and current waveforms, which can lead to circuit breakers detecting an overload condition, resulting in nuisance tripping.

The other potential causes, while they can impact electrical systems, do not necessarily lead to the same effects caused by harmonics. For instance, voltage sags often result from increased demand or an event causing a drop in voltage, transient voltages are short spikes that can affect equipment but typically do not cause the prolonged overheating seen with harmonics, and electrical surges can cause immediate damage but are less related to consistent operational failures like overheating.